I nursed my first daughter for 27 months. My second is 19 months, and I am still nursing her almost full time. There is absolutely nothing wrong (and many things that are beneficial) to "extended" nursing.
That being said, both my girls got positively chomp-happy while they were teething. I had to get the nipple shields to protect my breasts from my clothing (and to allow them to heal) because at one point I had three different tooth-cuts on each nipple-area. The only thing that worked for me was to lightly flick the nose while firmly saying NO BITING, then removing her from the breast to sit up-right for a minute.
Sometimes I had to stick my finger in the side of her mouth to pry the jaws apart first, so she didn't bite down then scrape her teeth off the nipple without opening her jaws.
EDIT: Make sure that the boyfriend gets "play time" as well! Breasts DO have more than one purpose...